Rhizonema

Rhizonema is a genus comprising a single filament-forming cyanobacterium that lives almost exclusively inside lichens. Rhizonema was first recognised as a distinct, exclusively lichen‑forming lineage in 2009, when molecular evidence showed its photobionts were unrelated to free‑living Scytonema and merited their own genus and family. It is currently the only member of the family Rhizonemataceae, with a single accepted species, Rhizonema interruptum. The cyanobacterium forms blue-green to yellowish filaments composed of rectangular cells (with nitrogen-fixing cells called heterocysts) and produces true, Y-shaped branches, which are features that distinguish it from superficially similar photobionts. Found in lichens from Europe to East Asia, it occupies humid forest habitats on bark, mosses and shaded rock.